Force rebalance accelerometers which include one or more electromagnet assemblies for bringing about a so-called null position in response to an acceleration or force input are known. Exemplary accelerometers are disclosed in the following U.S. patents, all of which are incorporated herein by reference:
U.S. Pat. Nos. 4,182,187; 4,250,757; 4,394,405; 4,399,700; 4,400,979; 4,441,366; 4,555,944; 4,555,945; 4,592,234; 4,620,442; 4,697,455; 4,726,228; 4,932,258; 4,944,184; 5,024,089; 5,085,079; 5,090,243; 5,097,172; 5,111,694; 5,182,949; 5,203,210; 5,212,984; 5,220,831; 5,488,865; 5,557,044; 5,587,530; and 5,600,067.
An exemplary prior art force rebalance accelerometer is shown in FIG. 1 generally at 10. Accelerometer 10 includes one or more magnetic assemblies 12 and a proof mass assembly 14. The magnetic assemblies generally include a permanent magnet 16 for developing a magnetic flux, and an excitation ring or flux concentrator 18 which receives magnet 16 and directs the magnet flux as will become apparent below. The excitation ring is typically formed from a material having a relatively high permeability, such as Invar, to enable a magnetic return path to be formed for the magnet flux. In the illustrated example, excitation ring 18 is generally cylindrical in form and includes a base 20, an outer wall 22 joined with the base, and an upper surface 24. Upper surface 24 is defined between an outer edge 26 and an inner edge 28. The inner edge defines an opening 29 into an interior region inside of the excitation ring and within which magnet 16 is received. A plurality of electrical leads 30 are provided and extend from the outside world into the flux concentrator through openings (not specifically shown) formed therein. The openings, most often, are provided in the base of the flux concentrator and are filled with epoxy to fix the leads therewithin. The electrical leads operably connect with circuitry which enables the accelerometer to operate as described below. Exemplary outside world circuitry includes position detector circuitry for measuring electrical indicia of an acceleration-induced change in proof mass assembly 14, as will become apparent below. Exemplary position detector circuitry is disclosed in U.S. Pat. No. 4,634,965, which is incorporated herein by reference. Such outside world circuitry also enables electric current to be delivered to the proof mass assembly as will become apparent below. Electrical leads 30 also extend through a pair of apertures 32, 34 in upper surface 24 and operably connect with the proof mass through an electrical connection described below.
Turning attention to proof mass assembly 14, such includes a proof mass 36 having a pair of bobbins, one of which is shown at 38. Another bobbin is connected on the other side of proof mass 36 and is not specifically shown. Each bobbin supports electrically conductive structure in the form of torque coils 40 which are electrically connected with the outside world through electrical leads 30. Proof mass assembly 14 also includes a mounting ring 42 and a pair of flexures 44, 46 which provide a physical and electrical interconnection between proof mass 36, mounting ring 42, and electrical leads 30. Flexures 44, 46 define a hinge having a hinge axis about which proof mass 36 can rotate in response to an acceleration force. It is precisely this rotation or movement which is detected and acted upon by the outside world electrical circuitry and which enables calculation of an acceleration.
The nuances of the above-described accelerometer operation are described in great detail in many of the patents incorporated by reference above. In addition, the reader is referred to a text book entitled "Modern Inertial Technology, Navigation, Guidance and Control" by Anthony Lawrence and published by Springer-Verlag, the disclosure of which is expressly incorporated by reference herein. Briefly described below, however, are some operational characteristics which are helpful to understand the present invention.
The above-described accelerometer has three principal accelerometer axes which are of interest. The hinge axis A, mentioned above, defines an axis about which the proof mass can rotate in response to an acceleration. An input axis B defines a direction along which acceleration is desired to be measured. A pendulous axis C is also defined by the proof mass. The hinge (flexures 44, 46) supports proof mass 36 and desirably has a low rotational stiffness about hinge axis A, so that it does not restrain the proof mass or pendulum in its response to the acceleration it is supposed to sense. It is also desirable that the hinge have a high stiffness about input axis B and pendulous axis C so that it resists buckling, twisting, or bending in response to accelerations either along hinge axis A or pendulous axis C. Moreover, it is desirable that the hinge lie in the plane containing the proof mass center of gravity so that accelerations along the pendulous axis do not produce so-called cross-coupling errors. A cross-coupling error relates to an accelerometer's sensitivity to an acceleration along an axis other than the input axis. Such errors can be induced, for example, if the hinge is twisted or misaligned in its plane.
In operation, an acceleration force is experienced along input axis B which causes proof mass 36 to rotate about hinge axis A and toward one of magnet assemblies 12. Upper surfaces 24 of magnet assemblies 12 form, together with metallized portions of proof mass 36, a so-called capacitive pickoff. When the proof mass is centered between the two magnet assemblies, the two capacitances are equal. When an acceleration displaces the proof mass as mentioned above, the distance between the proof mass and the respective magnet assemblies changes, which in turn, causes one of the capacitances to increase while the other capacitance decreases. The capacitors are incorporated into circuitry which provides an output signal which is indicative of the displacement of the proof mass. The output signal is utilized in a servo system which generates an electrical current which passes into the torque coils by way of electrical leads 30. The current in the torque coils interacts with the magnetic field or flux from the magnet assemblies and generates a restoring force which restores the proof mass to its centered or null position between the magnet assemblies. The magnitude of the current necessary to return the proof mass to its null position gives an indication or measure of the acceleration experienced by the proof mass.
It is highly desirable to provide the restoring force at the effective center of mass of the proof mass for a number of reasons. Doing so avoids creating a rotation force on the hinge about an axis other than the hinge axis which, over a period of time or under high G-forces, can cause the hinge to deform. Additionally, collocating the restoring force with the proof mass effective center of mass reduces cross-coupling errors and non-linearity. Additionally, by not collocating the restoring force and the proof mass effective center of mass, a second degree of freedom is imparted to the proof mass which cannot adequately be servoed which, in turn, impacts the G range of the accelerometer.
Contributions to the proof mass's effective center of mass are made by not only proof mass 36 itself, but the area cut away to form flexures 44, 46 as well. The result of the flexures' contribution is that the effective center of mass is shifted or displaced from the center of the proof mass assembly 14, along the pendulous axis and away from the flexures. While either the restoring force or the proof mass center of mass can be moved to effectuate co-location of the same, it is generally easier to move the restoring force.
One practice in the past, with respect to relocating the restoring force, has been to provide a notch 48 (FIG. 1) in the excitation ring or flux concentrator. The provided notch is disposed between apertures 32, 34 through which electrical leads 30 extend. Notch 48 serves to redirect the magnetic flux produced by magnet 16 so that the restoring force is moved positively along the pendulous axis to coincide with the proof mass effective center of mass. Yet, a number of problems are presented by this approach. First, notch 48 creates a pair of thinned regions 50 in excitation ring 18. Because the magnetic flux avoids the gap defined by notch 48, it is redirected and concentrated toward the thinned regions 50. High concentration of magnetic flux in regions 50 can cause magnetic saturation effects which, in turn, degrade scale factor thermal hysteresis and generate long term drift errors. Needless to say, if the restoring force and the proof mass center of mass are not co-located, all of the problems discussed above come to bear on the accelerometer.
Additionally, manufacture of the accelerometer illustrated in FIG. 1 is necessarily complicated because access to the electrical leads is hindered by apertures 32, 34 and thinned regions 50. More specifically, the excitation ring structure essentially leaves the apertures (not specifically shown, but disposed in base 20) through which the electrical leads extend in a blind construction area. The electrical leads can be the source, after manufacture, of production failures thereby requiring re-work or re-potting with epoxy material. Because access to the leads is hindered by the existing excitation ring structure, such re-work is necessarily hindered and complicated.
This invention arose out of concerns associated with providing rebalancing accelerometer constructions and methods having improved scale factor performance, with better producibility and lower cost.